We have launched a new research program for the evaluation of fission product and actinide release behaviors in the severe accident, focusing on the chemical forms by improving the severe accident analysis code in response to the strong needs for reinforcement of nuclear safety as well as for implementation of the 1F decommissioning R&D project. In the research program, the CORSOR-M model incorporated in THALES-2 code will be improved through implementing the effects of chemical form and high-temperature chemical interaction between fuel and materials based on the investigation of the release kinetics and chemical stabilities of fission products and actinides.